Mikrotik Switch for separating individual clients traffic

Hi there,

I have a slight issue with one of my client’s location. So after their ISP upgraded to Fiber Optic things are getting crazy, basically there are STBs which are connected through individual LAN cables directly to the ISP Modem/Router combo. The problem is that if the STBs are not connected to the ISP Modem/Router combo (for example connect it to the Router), it will have an error message “no signal detected”. Now I also need APs running along the same cable as the STBs (it was working fine until the ISP changed all the STBs when everything was connected to the Main Router), but this only works if the Individual Cables are connected to the Main Router, but then the STBs gets no signal detected error. The STBs are those IPTV with Ethernet port. The ISP also locks the modem and disables bridging, VLAN, etc so the modem is basically fixed.

Setup with STB working but APs not working:
ISP Modem/Router ↔ Individual Cables ↔ Switch ↔ STB, APs, etc

Setup with APs working but STBs not working:
ISP Modem/Router ↔ Main Router ↔ Individual Cables ↔ Switch ↔ STB, APs, etc

I was wondering if it’s possible to get a Mikrotik RB260GS switch and set it up as such:
ISP Modem/Router ↔ Main Router ↔ Mikrotik Switch (also connected to the ISP Modem/Router ↔ Individual Cables ↔ Switch ↔ STB, APs, etc
Port 1 - ISP Modem/Router
Port 2 - Main Router (prefer not changing the current setup)
Port 3-5 - Individual Cables with APs and STBs connected to it
And then set it up manually so that each client are routed to either Port 1 or Port 2 (not based on ports).

I only know bridging works between ports, but will it work between ports and clients combo? Or is there another way to solve this? Installing new LAN cables would be best but it is impossible since it will destroy much of the location. Thanks so much!